The present invention relates to a joint dewatering zone of wires for dewatering a web to be formed in a two-wire papermachine having a first wire loop and a second wire loop, in which zone a first forming roll is situated inside the second wire loop and in which zone other dewatering means are situated after the first forming roll inside the first wire loop.
The present invention is applicable also to board machines, and the term "papermachine" signifies both papermachines and board machines in this context.
The invention is especially intended for two-wire machines wherein between a headbox and a two-wire former there is a single-wire initial section where the web obtains a suitable felting degree whereafter dewatering of the web takes place within the two-wire section, which starts at a point where the second wire loop, guided by the first forming roll, joins the first wire loop forming the said single-wire initial section.
The purpose of the two-wire formers described above is to obtain a better formation and to eliminate unequal-sidedness in paper by draining water out of the opposite sides of the web through both wires. This affects uniform distribution of additives in the web, such as fillers, and of fines in the web. Further, an improved dewatering capacity is achieved by means of two-wire formers.
The examples of the constructions mentioned above, are shown, for example Finnish Patent Applications Nos. 820742 and 820743 (Valmet Oy), Finnish Patent Application No. 813276 and U.S. Pat. No. 4.176.005 (Escher Wyss GmbH) as well as German Pat. No. 3.107.730 (J. M. Voith GmbH).
The above-mentioned two-wire formers have the drawback that the control of dewatering is poor and abrupt changes in dewatering rate can not be controlled, which results e.g. in undesirable removal of fillers added to the stock and of fine fibres too, as well as in a deteriation of web quality.